Not applicable.
(1) Field of the Invention
This invention generally relates to radiofrequency (RF) antennas and more particularly to RF antennas that have a compact form.
(2) Description of the Prior Art
Conventional antennas radiate RF energy from a metallic conductor. The efficiency of such an antenna depends upon its length and configuration. Antennas that are approximately one-quarter wavelength (xcex/4) for current fed antennas and one-half wavelength (xcex/2) for voltage fed antennas or an integer multiple thereof can be tuned to have a low VSWR with a gain that is a strong function of antenna length. Conversely, as antennas become shorter they have lower gain. When the length becomes shorter than a single quarter or half wavelength, VSWR increases, and antenna efficiency decreases.
For variable frequency applications it is typical to design an antenna for a center frequency and to use various tuning methods to match the characteristic impedance of the radiating element or elements to a predetermined transmitter output impedance. Marine vessels antennas often cannot accommodate quarter-wave or half-wave antennas due to space restrictions. So the antenna radiating element is merely a stub that attaches to a tuning circuit. Such stubs can be difficult to tune and have little or no gain. Marine vessels, also incorporate one or more antenna masts that carry a number of diverse antenna structures. For such applications an antenna design must provide adequate gain within available space and must be capable of operating with physically proximate antennas at other frequencies. Antennas with short radiating elements typically interact in arrays.
Plasma antennas constitute another type of radiating structure. For example, U.S. Pat. No. 3,544,998 (1970) to Vandenplas discloses a plasma coated antenna. An expandable sheath consisting almost entirely of positively charged ions acts electrically like a vacuum to isolate the antennas from a layer of plasma which encompasses the antenna. The plasma layer may be maintained over the antenna by a suitable container. The antenna may be selectively tuned by varying either the thickness of the sheath or the density of the plasma.
U.S. Pat. No. 3,914,766 (1975) to Moore discloses a pulsating plasma device. This device has a cylindrical plasma column and a pair of field exciter members disposed in spaced parallel relationship to the plasma column. Means are also provided for creating an electrostatic field through which oscillating energy is transferred between the plasma column and the field exciter members.
Still other antenna structures exist. For example, United States Statutory Invention Registration No. H653 (1989) of Conrad discloses a superconducting, superdirective antenna array. A superconductive material is employed for the elements of the array which are arranged in a uniform half-wave dipole having a low ohmic resistance and a very high radiation efficiency. The superdirective antenna array is a linear array with element spacing of less than xcex0/2 where xcex0 is the center frequency of the dipoles. A dielectric window directs radiation of a very high directivity from the superconducting, superdirective antenna array.
U.S. Pat. No. 3,665,476 (1972) to Taylor discloses a receiving antenna for submarines. Tunnel diodes are inductively coupled to a plurality of ferrite rods by a coupling link. The tunnel diodes are back biased circuit to establish operation in the negative resistance region. Bias current and coupling are adjusted to provide cancellation of the major portion of the ferrite core losses and cover losses of the main turning winding.
Each of the foregoing disclosed antenna structures has certain disadvantages. Specifically, each generally tends to operate at a particular frequency, not over a wide bandwidth. Moreover each usually requires use of significant space and therefore is not readily adapted for installation on an antenna mast or like supporting structure in a confined volume. Finally when such conventional antennas are located in an array, they tend to be interactive in the far field radiation. What is needed is an efficient, tunable, compact antenna structure that has a wide bandwidth and that operates independently of far field radiation from adjacent antennas in an array on a common antenna mast, particularly on marine vessels.
Therefore it is the object of this invention to provide an antenna that is compact in design and adapted for use in a variety of applications.
Another object of this invention is to provide a tunable antenna that is compact in design and is adapted for use in a variety of applications.
Still another object of this invention is to provide an antenna that provides improved radiation at lengths less than a quarter-wavelength or half-wavelength of the frequency being radiated.
An antenna constructed in accordance with this invention includes a confined plasma column that extends along an axis and that is characterized by a natural resonance frequency. A modulator applies an ac field to the confined plasma column at a frequency essentially corresponding to the natural resonance frequency whereby the plasma radiates RF energy at the frequency of the ac field.
In accordance with another aspect of this invention, an antenna array comprises at least first and second plasma antennas. The first plasma antenna comprises a first confined plasma column that extends along a first axis and is characterized by a first natural resonance frequency. A modulator applies an ac field to the confined plasma column at a frequency essentially corresponding to the first natural resonance frequency. The second plasma antenna comprises a second confined plasma column extending along a second axis. The second plasma column is characterized by a second natural resonance frequency that is different from the first natural resonance frequency. A modulator applies an ac field to the second confined plasma column at a frequency essentially corresponding to the second natural resonance frequency. When the first and second antennas are mounted in an array, the antenna with the much lower natural plasma frequency is unaffected by radiation from the other antenna.